EQUILIBRIUM RELATIONSHIPS ON THE LIQUIDUS SURFACE IN PART OF THE MnO‐Al ₂ O ₃ ‐SiO ₂ SYSTEM*

Abstract

A bstract The minerals found to have a stability range on the liquidus surface in the system MnO‐Al 2 O 3 ‐SiO 2 are cristobalite and tridymite, SiO 2 ; mullite, 3Al 2 O 3 . 2SiO 2 ; corundum, Al 2 O 3 ; rhodonite, MnO.SiO 2 ; tephroite, 2MnO.SiO 2 ; galaxite, MnO.Al 2 O 3 ; spessartite, 3MnO.Al 2 O 3 .3SiO 2 ; and a new compound, 2MnO.2Al 2 O 3 .5SiO 2 . The quintuple points and boundary lines found in that part of the MnO.Al 2 O 3 .SiO 2 system bounded by the systems 3MnO.Al 2 O 3 .3SiO 2 ‐SiO 2 , 3MnO.Al 2 O 3 .3SiO 2 ‐Al 2 O 3 , and Al 2 O 3 ‐SiO 2 have been determined. Quintuple points and boundary lines involving the area inclosed by the systems 2MnO . SiO 2 ‐3MnO . Al 2 O 3 . 3SiO 2 , 3MnO . Al 2 O 3 . 3SiO 2 ‐SiO 2 , and 2MnO.‐SiO 2 ‐SiO 2 are indicated. Isotherms and isofracts (lines of equal refractive index) are given for the portion of the system investigated. X‐ray data show that crystals of the compound 3MnO.Al 2 O 3 .3SiO 2 have the same structure as the garnet mineral spessartite (manganese garnet). The compound 2MnO. 2Al 2 O 3 . 5SiO 2 , which is shown by X‐ray data to have a basic structure similar to cordierite, 2MgO.2Al 2 O 3 .5SiO 2 , seldom crystallizes from the glass, but a third substance which has a high extinction angle crystallizes readily from the reheated glass. These high extinction angle crystals are replaced by 2MnO .2Al 2 O 3 .5SiO 2 when a proper heating cycle is employed, and they are not found again until the product is melted and crystallization is repeated.